The wiki discusses the ship roles, and does a fine of doing it. And it was just posted here recently, but briefly:
Support - stays in the back and won't move until enemy offensive ships are all dead (Interceptors, Assault, and Capital). If a ship comes in range, however, they will shoot. I like to use at least one Support which I load with sensors, but will probably give that role to my Capital now (or again, as I had just recently switched to using a Support ship for this). Or just not have high sensors within the fleet itself.
Defensive - stay with the ship they are assigned to protect and target ships targeting their protected ship first. Guardians prioritize protecting Support, so if you have Support in your fleet, Guardians are not moving forward either. Escorts, on the other hand, give priority to protecting the Capital ships, so WILL move forward. It is important to note that the heavy guns of your enemy, offensive ships sized smaller or bigger, will target your Escorts first, once your Escorts are in range. High armor would not be a bad idea for your Escorts. If you do not use Support ships, thrusters on your Guardians might be a good idea too, because they will then be prioritizing your Capital for defensive targeting, and will be soaking hits from enemy Interceptors if they can catch up to your Capital.
Offensive - move forward and have no designated defensive role. Assaults and Capitals both target enemy Escorts, then Capitals, then Assault ships, before other stuff. In your entire fleet, only Guardians and Supports target Interceptors first. Everything else will only target Interceptors if nothing else is in range or after destroying several lines of other ships first (but since Interceptors are first in line, your ships should be able to take shots at them before moving on to prioritized targets).
Of course, you could just make all your high armor ships escorts and high offensive ships capitals, but where is the fun in that?
edit: Yes, thanks again, max tactical speed is good to know. Difference of 1.2. Got it.